Ground fault interrupter with pulsed neutral-to-ground fault detector

ABSTRACT

Apparatus for monitoring an alternating-current circuit having a differential current transformer to sense line-to-ground faults includes a pulsed ringing circuit for evidencing freedom of the alternating-current circuit from neutral-to-ground faults that could reduce the sensitivity of the line-to-ground fault detector.

United States Patent 1 [111 3,794,884 Sircom Feb. 26, 1974 GROUND FAULTINTERRUPTER WITH I PULSED NEUTRAL-TO-GROUND FAULT [56] References CitedDETECTOR UNITED STATES PATENTS [75] Inventor: Richard C. Sircom,Scarborough, 3,638,072 1/1972 Kobayashi 3l7/l8 D Ontario, Canada3,597,656 8/1971 Douglas 317/18 D 3,713,003 ll973 B h 317 l8 D [73]Assignee: Federal Pacific Electric Company, en am Newark PrimaryExaminer--J. D. Miller [22] Filed: May 29, 1973 Assistant Examiner-PatSalce [21] Appl. No.: 364,890 [57] ABSTRACT 63 f Application DataApparatus for monitoring an alternating-current cir- 1 commuanon'm'panof 320355 cuit having a differential current transformer to sense 1973'lineto-ground faults includes a pulsed ringing circuit for evidencingfreedom of the alternating-current cirg F' 'i" 317/18 317/27 4;: cuitfrom neutral-to-ground faults that could reduce H gz' R the sensitivityof the line-to-ground fault detector.

' 317/52 7 Claims, 1 Drawing Figure PAH-1mm rtezelsu l 1 GROUND FAULTINTERRUPTER WITH PULSED NEUTRAL-TO-GROUND FAULT DETECTOR Thisapplication is a continuation-in-part of my appli cation Ser. No.320,855, filed Jan. 4, 1973, and represents an improvement thereof.

This invention relates to apparatus for monitoring an alternatingcurrent circuit for detecting neutral-toground faults, and to combineddetectors for both lineto-ground faults and neutral-to-ground faults.

BACKGROUND OF INVENTION The above identified application discloses aline-toground leakage detector involving a differential currenttransformer, (commonly called a DCT) together with means for detecting ameutral-to-ground fault of such low impedance as to impair significantlythe sensitivity ofthe line-to-ground fault detector. In principle, theleakage current resulting from a fault between the line conductor andground in an alternating current circuit should return to .thealternating current supply by way of the ground-return path. However, incase of a lowimpedance fault'from neutral-to-ground, a fraction of theline-to-ground leakage current would return to the a-c source via theneutral. That fraction of the leakage current would not contribute tothe unbalance in the DCT, of the line-to-ground fault detector which,accordingly would not respond properly to the full lineto-ground leakagecurrent.

In my pending application, a DCT is excited by an oscillator where'theline and-neutral conductors serve as primary windings of the DCT. Incase of a neutral-toground fault of low enough impedance to desensitizethe DCT of the line-to-ground fault detector, a ground loop is createdthat loads the DCT-oscillator circuit. That ground loop consists of theneutral conductor, the ground-return path, the grounding connection ofthe neutral at the a-c supply and the neutral-to-ground fault. The Q ofa winding on the DCT is reduced, reducing the outputof the oscillatorand causing tripping of a circuit interrupter.

Under an established standard, any value of line-toground fault of24,000 ohms or less on a l20-volt circuit would produce a leakagecurrent of 5 milliamperes or more, regarded as the hazard level of theline-toground fault detector. Correspondingly, a ground-loop impedanceof four ohms or less is regarded hazardous, unduly reducing thesensitivity of the line-to-ground fault detector. However, it'has beenfound difficult to maintain the amplitude of the oscillator at thedesired level, particularly when proper operation is required over atemperature range of 65 C to -35 C as in the above-mentioned standard.

SUMMARY OF THE INVENTION sisting of the output winding andthe capacitoris excited by a pulse of nearly-constant amplitude, causing the resonantcircuit to ring. Under normal conditions, when there is noneutral-to-ground fault, the ringing decays only a little during a shortperiod after each pulse. In case a ground loop develops due to aneutralto-ground fault, the ringing is damped during the same period.This effect is utilized in detecting the appearance of aneutral-to-ground fault of such a valve as to desensitize significantlythe line-to-ground fault detection means.

In common with my above identified application, the same DCT and thesame amplifier are used in both the line-to-ground fault detector andthe neutral-to-ground fault detector, leading to important economies.Failure of the pulsing circuit or a break in the DCT output winding havethe same effect as the damping that represents a neutral-to-groundfault, imparting a large measure of fail-safe performance.

Coupling of various signal points in the circuit to cutoff biasingpotential is included, having the effect of suppressing spuriousoperation of the circuit when power is first turned on and until thewhole circuit reaches stabilized operating conditions.

The nature of the invention and the foregoing and other novel featuresand advantages will be better appreciated from a review of the followingdetailed description of a presently preferred embodiment shown in theaccompanying drawings.

The single FIGURE of the drawings is a wiring diagram of aline-to-ground fault detector, integrated with a novel neutral-to-groundfault detector, the diagram including rectangles A through Hrepresenting signals appearing under various conditions at the indicatedpartsof the wiring diagram.

In the drawing, line L and neutral N extend from terminals L-l and N-lat the supply end of an alternating current branch circuit to terminalsL-2 and N-2 at the load end of the alternating current circuit.Typically, the a-c frequencyis Hz. In conventional manner, the neutral Nhas a ground 11 adjacent supply terminal N-l. Line conductor L is seento include an interrupter having contacts .10 and a trip coil 12. Incommon practice interrupter l0 12 is a circuit breaker having ashunt-trip coil for causing the contacts to open, additional to anyoverload release that is included, such as an overcurrent responsivebi-metal and an overcurrent responsive magnetic tripping means.

The circuit illustrated includes a full-wave diode bridge 16 havingalternating current supply terminals 18 and 20. Terminal 18 is connectedto line conductor L by way of trip coil 12 (rather than directly) as amatter of convenience. The current drawn by the bridge is far lower thananything that would activate the tripping machanism. Terminal 20 isconnected to the neutral conductor N. Direct-current output terminals 22and 24 are connected to a series circuit including zener diodes 28 and30 and a dropping or regulating resistor 32. Regulated d.c. potentialappears between positive line 34 and negative line 36, filtered bycapacitor 38. A mean d-c potential point 40 midway between lines 34 tand 36 provides what amounts to a do ground or comthereby causesshort-circuiting of the bridge so that an energizing circuit for thetrip coil extends from neutral conductor N via terminals 20 and 18 totrip coil 12 and to line conductor L.

A differential current transformer 46 includes a toroidal core 48,advantageously compact and made of ferrite, that is threaded byconductors L and N in such a sense that their respective fluxes due toalternating current supplied to a load at terminals L-2, to N-2 aremutually cancelling and ideally no magnetic flux is developed in thetoroidal core. A secondary winding 50 on the toroid develops output inthe event that there is any unbalance in the currents carried by theconductors L and N. Operational amplifier 52 amplifies any 60-cyclesignal from winding 50 resulting from unbalance of the currents in theconductors L and N. Secondary winding 50 of the differential currenttransformer is connected by a resistor 56 to the non-inverting inputterminal of operational amplifier 52. Operational amplifier 52 alsoserves as a pulse generator for triggering SCR 42 to operatetrip coil12. The non-inverting input of amplifier 52 extends to the d-c groundthrough resistor 56 and coil 50, providing an operating-point bias forthe amplifier. Resistors 58 and 60 are connected in series between theoutput of amplifier 52 and the d-c ground, and the junction of resistors58 and 60 is connected to the inverting input terminal of the amplifier,thus providing a relatively high stabilized closed-loop gain for d-c andlow-frequency signals, a gain of 1000 for example. Resistor 62 andcapacitor 64 in the megative-feedback path of the amplifier reduces thegain of the amplifier at higher frequencies. In an example, the gain isapproximately 30m KHz.

ln the event of a fault F- l between line L and ground, the current inneutral conductor N is less than that in line conductor L by the amountof the leakage current of fault F-l. The resulting 60 Hz output isamplified by 1,000 (in this example) and fed through a low-pass filtercomprising resistor 66 and capacitor 68 in series, to remove highfrequency content. The wave-form of this signal is shown in rectangle H.Any d.c. offset in the amplifier output is decoupled'by capacitor 70which drives a signal voltage doubler including diode rectifiers 72 and74, and capacitor 76. The doubler is referenced from a negative biaspoint established at the junction of resistors 78 and 80. This junctionis bypassed to negative bus 24 by capacitor 82.

In the absence of a fault F-l between line L and ground, the d-c outputof the doubler would be substantially zero and therefore, the cathode ofdiode 72 will be negative with respect to the d-c ground of the circuit.A diode 84 is connected between the output of voltage doubler 70, 72,74. The cathode of diode 84 is reverse biased. On the other hand, when afault F-l appears that is sufficiently serious to warranttripping of thecircuit interrupter, diode 84 becomes forwardbiased, so that a positived.c. voltage is developed across resistor 56 in series with the lowresistance of secondary winding 50. This is amplified by the factor of1,000 in the present example, driving the output of the amplifierstrongly positive. This positive-going output is coupled back to thenon-inverting input, since diode 84 is now conducting. This positivefeedback insures a snap" action, driving the amplifier almost to thevoltage of the positive bus 34. A zener avalanche diode 86 is connectedbetween resistors 66 at the output of amplifier 5'2 and resistor 88which is connected to the negative bus 24. The junction of resistor 88and diode 86 is connected to the gate 44 of SCR 42. When the amplifieroutput approaches the voltage of the positive bus 34, the avalanchevoltage of diode 86 is exceeded and a triggering voltage reaches gate 44of the SCR. When this occurs, the d-c output of bridge .16 isshort-circuited and consequently an alternating current energizingcircuit for trip coil 12 develops from line L through theshort-circuited bridge to neutral conductor N. Consequently, when ahazardous fault F-l develops between line conductor L at the load sideof DCT 46, trip coil 12 is energized and causes interrupter contacts 10to break the circuit from supply terminal L-l to load conductor L. Ahazardous fault F-l under certain Underwriters Laboratory standards is 5milliamperes or more which in a l20-volt circuit, is 24,000 ohms orless.

As indicated above, the sensitivity of DCT 46 to respond to leakage fromline L to ground via fault current path F-l is diminished in case aneutral-to-ground fault F-2 should occur. It is recognized that aclosed-loop circuit develops when fault F-2 occurs, includingthe .groundconnection 11 adjacent supply terminal N-l,

the neutral conductor N, fault F-2, and the groundreturn path. Thatreturn path may be a grounding or shield conductor or green wire. Theclosed-loop circuit may be called a ground loop. Upon occurrence of alow-impedance neutral-to-ground fault F-2, a fraction of the faultcurrent from line L-l to ground via fault F-l which reaches the groundconductor would then return to the neutral conductor by way of fault F-2and return to supply'termina l N-l through the DCT 46. That represents aloss of sensitivity of the detector to the actual value of the currentflowing in fault F-l. The following provision is made for causing thetrip coil 12 to open the interrupter l0 incase a fault F -2 develops ofsufficient magnitude to reduce unduly the sensitivity of thedifferential current transformer.

Ringing pulses are impressed on the differential cur-' rent transformer46. For this purpose, the output winding 50 is utilized in the ringingcircuit, although a separate winding might be made to serve for the samepurpose. Transistor 90 with a grounded emitter has its base driven viaresistor 92 by a 60 Hz signal supplied from line L via trip coil 12.When this signal is negative, transistor 90 is cut off, and itscollector is at a d-c level of a fraction of a volt (for example) as setby resistors 94 and 96. When its base is driven positive, transistor 90saturates, driving its collector to the potential of the d-c ground. Inthis way a small negative-going pulse is applied to winding 50 of DCT 46via capacitor 100. This signal is represented in rectangle A. Winding 50and capacitor '100 form a resonant circuit which rings at about 5 KHz(for example) during'the positive halfcycles of the 60 Hz wave. Theringing circuit includes winding 50, capacitor 100, and transistor 90 inits saturated condition. 7

Under normal conditions (with no fault F-2 from neutral-to-ground) asubstantial ringing amplitude remains at the end of each positivehalf-cycle. However, as soon as transistor 90 is cut off, the value ofresistor 96 effectively in parallel with'resistor 94 is introduced inseries with capacitor 100. This reduces the Q of the resonant circuit toa very low value so that the oscillations cease almost instantly. Theresulting wave-form is' shown in rectangle B at the top of the drawing.

The signal B is amplified moderately by amplifier 52, for example by afactor of 30, along with any 60 Hz voltage due to line-to-ground leakagein path F-l. For fault signals below the trip level, the combinedsignals on the amplifier output are as shown in rectangle C. Thehigh-frequency components are separated by highpass filter includingseries'capacitor 102 and shunt resistor 104. Under normal conditions,the output of this filter is the signal represented in the rectangle D.This signal is fed to the base of transistor 108. Diode 106 matches thebase-emitter diode path of transistor 108, providing a discharge pathfor capacitor 102.

The collector of transistor 108 is supplied with alternating-currentvoltage through resistor 110, the same source as that which drivespulsing transistor 90. Thus, when the collector of transistor 108 ispositive, it is pulsed into saturation at a 5 Kl-lz rate, producing thewave form represented in rectangle F. This is a half sign-wave envelopeof sawtooth pulses of low average tor supply. These high frequency.pulses are by-passedto d-c ground by capacitor 82.Consequently,-there'is virtually no output from the voltage doubler 70,72, 74, as previously described.

In case a neutral-to-gr-ound fault F 2 should develop of such lowimpedance asto significantly reduce the sensitivity of DCT 46 to a'faultF- 1, then a. lowimpedance loop has come into being. This ground loopinvolves neutral conductor N, neutral-to-ground fault F-2, theground-return path and the ground connection 11 to neutral terminal N-l.This loop causes damping of the resonance that normally develops in thecapacitor 100 and winding 50, or stated in other terms, lowers the Q ofthe resonant circuit. The ringing pulse is damped, and appears asrepresented in rectangle E at the output of filter 102, 104.Consequently, transistor 108 will no longer be saturated for the 'fullduration of the positive half-cycle of its collector supply. This willallow the collector voltage to go strongly positive toward theend of thepulse as shown in thecurve of supply circuit having at least onelineconductor, a neurectangle G. This raises the mid-point of voltagedivider 78, 80 correspondingly, driving the non-inverting input ofamplifier 52 positive through resistor 116 and diode 84. Thiscauses"snap action triggering of SCR 42, as described above. I

Capacitors 68, 76 and 82 are returned to the negative bus 24, ratherthan to the d-c ground. At the ,time that the entire circuit is firstenergized, the negative bus 24 goes rapidly negative, and the oppositeelectrodes of these capacitors follow the bus momentarily. This preventsfalse triggering during the few milliseconds required for the amplifierto settle and establish normal operation.

Button 118 applies the usual test current to DCT 46 from line L to a-cground, for demonstrating the responsiveness of the circuit to a faultF-l. During this test and at any other time, if certain parts of thesystem should fail (such as connections to winding 50 'or a break inthat winding) then transistor 108 will not be saturated for any part ofthe positive half-cycle of the collector supply, and trip coil 12 wouldbe energized to open interrupter 10.

It is evident that differential current transformer 46 can be used withthe 5 Kl-lz pulsing and pulseresponsive systemapart from the parts ofthe circuit forming a system for detection of line-to-ground faults.Such a detector for faults F-2 could be used with a separate detector(with its own DCT) for detecting faults F-l. It is also apparent thatadditional line conductors 1 L can be included, with or withoutmodifications of the illustrated circuit.

The illustrated circuit is operative and stable over a wide frequencyrange. It is economical since it uses manyparts of the circuit forseparately testing for the two different faults F-l and R2 which are ofsuch widely different values. Moreover, partly because many circuitcomponents are used in common in both fault detection functions andpartly because the failure of many of the circuit componentsand-connections would result in automatic activation of the circuitinterrupter,

a high order of fail-safe operation is realized.

While-the foregoing illustrative embodiment'of the invention isexemplary, it will berecognized that varied rearrangements andmodifications as well as various applications of its novel features willbe made readily by those skilled in the art. Accordingly, the inventionshould be construed broadly in accordance with its full spirit andscope.

lclaim: I 1. Apparatus for monitoring an alternating currenttralconductor, and means forming a ground-return, wherein saidconductorsextend from supply terminals to load terminals and wherein theneutral conductor has a ground connection to the ground-return, themonitoring apparatus including neutral-to-ground fault monitoring meanshaving a differential current transformer having a core and primarywindings forming series segments of said line and neutral conductors soarranged that loadcurrent carried thereby tends to produce mutuallycancelling flux in said core, the differential current transformer beingdisposed between said ground connection and said load terminals,resonant means resonant at a higher frequency than thatof thealternating current supply circuit including a further winding linkingsaid core and a capacitor connected to said further winding, means forperiodically pulsing said resonant means, and fault-evidencing meanscoupled to said transformer for responding thereto in one .manner toindicatenormal operation when the reso nant means rings at least at aminimum amplitude for a prescribed period aftereach pulse of the pulsingmeans and for responding thereto differently to indicate abnormaloperation in the event of a fault between the neutral conductor and theground-return means at the load-terminal side of said transformercausing damping of 'the resonant means and decay of the-ringing belowsaid minimum amplitude during said period. 2. Monitoring apparatus inaccordance with claim, 1, wwherein said neutral-to-ground faultevidencing means and said resonant means include saidfurther winding incommon. a

3. Monitoring apparatus in accordance with claim 1, wherein said pulsingmeans is coupled to the alternating current supply circuit to besynchronized therewith.

4. Monitoring apparatus in accordance with claim 1, wherein saidfault-evidencing means includes a transistor pulsed into saturation atthe ringing frequency so long as the minimum ringing amplitude issustained.

5. Monitoring apparatus in accordance with claim 4, further includingtime-constant circuit means restraining response to the transistorforintervals only slightly longer than the period of the ringing frequency.

mon.

1. Apparatus for monitoring an alternating current supply circuit havingat least one line conductor, a neutral conductor, and means forming aground-return, wherein said conductors extend from supply terminals toload terminals and wherein the neutral conductor has a ground connectionto the ground-return, the monitoring apparatus includingneutral-to-ground fault monitoring means having a differential currenttransformer having a core and primary windings forming series segmentsof said line and neutral conductors so arranged that load currentcarried thereby tends to produce mutually cancelling flux in said core,the differential current transformer being disposed between said groundconnection and said load terminals, resonant means resonant at a higherfrequency than that of the alternating current supply circuit includinga further winding linking said core and a capacitor connected to saidfurther winding, means for periodically pulsing said resonant means, andfault-evidencing means coupled to said transformer for respondingthereto in one manner to indicate normal operation when the resonantmeans rings at least at a minimum amplitude for a prescribed periodafter each pulse of the pulsing means and for responding theretodifferently to indicate abnormal operation in the event of a faultbetween the neutral conductor and the ground-return means at theload-terminal side of said transformer causing damping of the resonantmeans and decay of the ringing below said minimum amplitude during saidperiod.
 2. Monitoring apparatus in accordance with claim, 1, wwhereinsaid neutral-to-ground fault evidencing means and said resonant meansinclude said further winding in common.
 3. Monitoring apparatus inaccordance with claim 1, wherein said pulsing means is coupled to thealternating current supply circuit to be synchronized therewith. 4.Monitoring apparatus in accordance with claim 1, wherein saidfault-evidencing means includes a transistor pulsed into saturation atthe ringing frequency so long as the minimum ringing amplitude issustained.
 5. Monitoring apparatus in accordance with claim 4, furtherincluding time-constant circuit means restraining response to thetransistor for intervals only slightly longer than the period of theringing frequency.
 6. Monitoring apparatus in accordance with claim 1,further including means coupled to the differential current transformerfor evidencing the appearance of a line-to-ground fault which causesflux unbalance in said core at the frequency of the alternating currentcircuit.
 7. Monitoring apparatus in accordance with claim 6, whereinsaid neutral-to-ground fault evidencing means and said line-to-groundfault evidencing means and said resonant means include said furtherwinding in common.